Interpretive Summary: Avian infectious bronchitis virus (IBV) is the etiologic agent of infectious bronchitis (IB), which causes an acute, highly contagious upper-respiratory tract disease in chickens, which affects the respiratory, reproductive, and renal systems. IBV is of considerable economic importance to the poultry industry because diseased birds characteristically have poor weight gain and feed efficiency, and increased production losses. In situ hybridization techniques allow for precise identification of nucleic acids in histologic sections and can detect replicating pathogens before serologic conversion of the host immune system. Using this techniques, we were able to detect replicating IBV in the lung and bursa following virus inoculation of chicken embryos.

Technical Abstract:
In situ hybridization and immunohistology were utilized to identify tissues infected in ovo with infectious bronchitis virus (IBV). Chicken embryos were inoculated in ovo with the Arkansas (Ark) serotype of IBV at 18 days-of-age. At 24, 48, 72 and 120 hours post-infection (h.p.i.), bursa, lung, spleen, heart and thymus were collected, fixed in 10% neutral buffered-formalin and paraffin-embedded. The digoxigenin (DIG)-labeled antisense S1 riboprobe detected viral mRNA in the cytoplasm of respiratory epithelial cells in the primary bronchus at 24, 48 and 72 hours h.p.i. Immunohistology detected viral antigens in epithelial cells of the parabronchi and bursal tissues at 24 and 48 hours, respectively. No viral mRNA or antigens was detected by in situ hybridization or immunohistology, respectively, in heart, thymus or spleen at any time point following inoculation. These results indicate that in situ hybridization can be useful in detection of IBV-infected chickens and understanding the pathogenesis and virulence of IBV infection.